a) Field of the Invention
The present invention preferably concerns a microphone, in particular a digital microphone, that is to say a microphone in which the signal which comes from the microphone transducer is already digitized in the microphone and the corresponding microphone signal is delivered digitally by the microphone, either by way of a cable or wirelessly, for example in a microphone receiver, pre-amplifier etc.
b) Problems Addressed by the Invention
High-quality audio data produce large amounts of data after A/D conversion. If, as is usual for example in studio technology, the procedure involves sampling an audio signal at 48 kHz with a word width of for example 16 bits, the result is a data stream of 768000 bit/s per channel, which in the case of a stereo channel already signifies more than 1.5 Mbit/s.
In order to protect the data generated in that way from transmission errors, redundancy is added in the form of additional bits which are used at the receiving end for error recognition and error correction. As a result the amount of data to be transmitted is considerably increased once again, depending on the respectively desired correction capacity.
In order to reduce the amount of data to be transmitted audio data compression processes, for example in accordance with the MPEG standard, are frequently used. They reduce the redundancy contained in the audio signal insofar as they process longer blocks of audio data in accordance with various processes. With an increasing block size the compression rate which can be achieved increases and thus the amount of data falls. Processes which suffer with losses and loss-less processes are known, the former achieving a higher compression rate.
That block formation results in the generation of a delay in the transmission system as a data block can only be processed when it has completely arrived. That applies both in regard to the transmitting and also the receiving end. That effect is unwanted in some uses, in particular in relation to studio applications in which microphones of the present kind are to be particularly used.
If the transmission path is at the beginning of an audio production chain, the reduction in redundancy in the audio signal is disadvantageous. Subsequent production steps could influence the audio signal in such a way that the quality thereof suffers markedly. In addition the free creative options of sound mixers would be restricted. Compression processes are therefore unwanted in high-quality transmission situations, that is to say for example in the studio, on the path from the microphone to the pre-amplifier, mixer, recording unit etc.
Compressed audio signals are more sensitive to transmission errors. Disturbing a few bits within a compressed data block can render the entire block useless. Therefore enhanced precautions are to be taken for error correction, whereby the amount of data to be transmitted increases again. With certain error scenarios, the reduction in the amount of data can be completely nullified by the necessary error protection and even made into just the opposite.
The above-described aspects cause the use of audio data compression procedures to be fundamentally problematical so that as far as possible the use thereof should be entirely abandoned, at least in terms of recording audio in a studio.
Present day systems for digital wireless radio transmission of audio data which must manage without compression processes or only with low compression factors operate in frequency ranges which admit high occupied bandwidths. The ISM bands at 900 MHz and 2400 MHz are popular. There the bandwidth occupied can be several MHz. It will be noted however that those frequency ranges are also used by many other radio systems. Prioritisation of given uses is not possible by virtue of authorisation regulations in those frequency ranges. The transmission of audio data from microphones to the receiver is accordingly in danger of being disturbed in an unwanted fashion at any time because other transmitters are possibly transmitting in the proximity, on the same frequency band.
Frequency ranges outside the ISM bands without general clearance are available for professional uses. Transmission would be possible there, without involving interference. It will be noted that historically, due to analog radio transmission, authorisation regulations which require low occupied bandwidths apply to those frequency ranges. Thus for example an occupied bandwidth of a maximum of 200 kHz is prescribed for radio microphones in the UHF range (in Europe).
High-quality and thus high data-rate digital transmission systems with slight transmission delay in such narrow bandwidths are however not available.